A few things here:
1. Mollies actually prefer slightly brackish water and contrary to popular belief do not therefore mix well with other FW species. They like a different water chemistry. Yes, you CAN keep them in a bog standard FW setup but they will be tolerating this at a certain level of stress. Ever wonder why so many people have mollies with Ich?
2. Neons are a schooling fish and prefer to be in a groups of at least 6. They will provide much more colour and motion than when kept in small numbers.
3. For hardy fish, you can't go wrong with platys or swordtails. They are ROCK solid fish, many people use them for cycling their tanks. Two words of warning though. They are total pigs and will eat as much as you give them and overeat themselves sick....and they breed like there's no tomorrow. Try and get either all males or all females unless you want to start dealing with numerous broods of fry in the near future. Even if you just get females they will likely come pregnant and you may get a brood, I did when I got mine. They are fun fish though and have distinct personalities.
4. You could get some Corydoras catfish for the bottom to help clean up. There are numerous varieties out there, the nice thing is they don't get too big...about 2". I personally have albino ones and they are great fun to watch. Whatever you do DO NOT get a pleco. They get WAAAAY to big for a 20g tank and will totally take it over. NOR a Chinese algae eater....nasty fish!
5. Other good community fish are: White cloud minnows, Zebra danios, all tetras (although they can be a bit finicky about water quality and stability and perhaps not described as hardy), barbs (if you get the less agressive species - or a large anough school that they don't terrorise the other fish), rasboras.
Your biggest consideration with your stocking, needs to be the numbers you put in the tank as it's only 20gal. I would personally pad out your tetra school to six, add two platies and get 4 of the smallest cory cats you can find. Add that to your 4 guppies and you'd be fully stocked and it would make a nice community tank! (Which spookily enough is very similar to my own in my 29gal !)
